Maybe the folllowing will help you get on the right path. It would be nice if you didn't have to call enforce_interface in all the child classes...

use strict; use warnings;

my $bar = Bar->new;

#------------------------
package Foo;

# implement these in the child class
# sub bing { }
# sub bang { }
# sub bong { }

sub enforce_interface {
    my $self = shift;
    for ( qw/bing bang bong/ ){  # require these methods
        die "$_ not implemented" unless $self->can( $_ );
    }
}

1;

#------------------------
package Bar;

sub new { return bless {}; }

# don't implement bing
sub bang { }
sub bong { }

use base qw/Foo/;
__PACKAGE__->enforce_interface;

1;
[download]

and when run:

bing not implemented at test.pl line 17.
[download]

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